Molecular surveillance of the antifolate-resistant mutation I164L in imported african isolates of Plasmodium falciparum in Europe: sentinel data from TropNetEurop

Abstract

Background

Malaria parasites that carry the DHFR-mutation I164L are not only highly resistant to sulfadoxine-pyrimethamine but also to the new antimalarial drug chlorproguanil-dapsone. The spread of this mutation in Africa would result in a public health disaster since there is a lack of effective alternatives that are both affordable and safe. Up to now, this mutation has only been described in Asian and Latin-American countries. The objective of this study was to assess the prevalence of this mutation in African isolates of Plasmodium falciparum that have been imported into Europe through travellers.

Methods

TropNetEurop is a network for the surveillance of travel-associated diseases and seems to cover approximately 12% of all malaria cases imported into Europe. Within this network we screened 277 imported African isolates of P. falciparum with the help of PCR- and enzyme-digestion-methods for the antifolate-resistant mutation I164L.

Results

The I164L mutation was not detected in any of the isolates tested.

Discussion

Continuous molecular surveillance of mutations in P. falciparum, as it is practised within TropNetEurop, is an essential tool for the understanding and early detection of the spread of antimalarial drug resistance in Africa.

Background

Drug resistant malaria is a major problem in malaria control. At the present, there are few antimalarial drugs available in endemic areas which are both cheap and save. Since resistance to chloroquine has spread across sub-Saharan Africa, several countries have switched their first-line drug to the antifolate sulfadoxine-pyrimethamine [1].

Soon after sulfadoxine-pyrimethamine was introduced to malaria-control programs in the late 1960s, resistance to this drug was noted [2]. The efficacy of sulfadoxine-pyrimethamine primarily depends on the sensitivity of the parasite to pyrimethamine [3]. The dihydrofolate reductase domain of Plasmodium falciparum (pfDHFR) is the target of this drug. DHFR catalyzes the reduction of dihydrofolate to regenerate tetrahydrofolate which is required for one-carbon transfer reactions and deoxythymidylate synthesis of the parasites.

The discovery of changes in codons of the pfDHFR-gene strongly indicated that single amino acid changes lead to observed resistance. Mutations at amino acid position 51, 59, 108 and 164 have been shown to be linked with resistance of P. falciparum to antifolate antimalarials. A scheme for evolution of resistance could be derived as a result of stepwise mutations starting with the S108N mutation, which was shown to be the optimal mutation in leading to both the decreased binding affinity for inhibitors and the retention of enzyme activity [4, 5]. Absolute resistance is conferred by the addition of I164L mutation in the quadruple mutant form (N108/I51/R59/L164). In this fourth mutation the enzyme is about 1,000-fold less sensitive to pyrimethamine [6].

The I164L mutation also plays an important role in the development of resistance to the more potent antifolate combination chlorproguanil-dapsone: while the triple mutant allele (N108/I51/R59) has no great impact on the sensitivity of this drug, parasites that carry the quadruple mutant allele are resistant [7, 8]. This highly potent quadruple mutation was described first in South East Asia, later also in South America [9–11]. There is a concern that continued sulfadoxine-pyrimethamine pressure, as well as the widespread use of trimethoprim-sulfamethoxazole for prophylaxis against opportunistic infections in patients with AIDS in Africa, is going to select these highly resistant alleles [12]. This would rapidly make sulfadoxine-pyrimethamine, as well as chlorproguanil-dapsone, ineffective in this region. For this reason continued surveillance of this mutation is needed to evaluate the prevalence, the distribution and the speed with which these populations might be selected.

TropNetEurop is a European surveillance system that covers approximately 12% of all imported malaria cases in Europe [13]. Travellers can be used as a highly sensitive surveillance tool to detect development of drug-resistance in endemic areas, as it is suggested that most of them carry a monoclonal Plasmodium strain. Initial data on molecular surveillance, not including the I164L mutation, have already been published elsewhere [14].

Since there are only limited data available on the prevalence of the I164L mutation throughout Africa, the purpose of this survey was to screen a sample of P. falciparum imported from Africa for this particular antifolate mutation.

Material and methods

Sampling

The European Network on Imported Infectious Disease Surveillance (TropNetEurop) has been established in 1999 and, in 2001, covered approximately 12% of all imported malaria cases in Western and Central Europe [13]. At the present 45 clinical sites in Europe are members of the network with more than 57,000 patients seen post-travel annually.

Within the infrastructure of TropNetEurop several member sites started in 2001 to collect blood samples from patients diagnosed with a P. falciparum infection. 10 μl of whole blood from each patient were dotted on Whatman 3 MM^® chromatography paper and air-dried at room temperature before initiation of treatment. The filter paper disks were sent to Munich for further preparation.

For this survey we screened all blood samples that were obtained between January 2001 and July 2002 from travellers returning from Africa with a microscopically confirmed P. falciparum infection.

Parasite DNA and Polymerase chain reaction

Parasite DNA was prepared from the dried blood spots on the filter paper by the Chelex method as described by Kain and Lanar [15]. Nested mutation-specific PCR was done as previously described for analysis of the DHFR 164 mutation site [16, 17].

Restriction fragment length polymorphisms

A volume of 4 μl of PCR product was incubated for three hours at a temperature of 37°C with the mutation specific restriction enzyme DraI according to the manufacturer's instructions (New England Biolabs, Beverly, MA, USA). To discriminate between the two variants of codon 164, the 522 bp PCR product was digested with DraI to detect the leucine mutation (143 bp). The digestion products were separated by electrophoresis in a 1% SeaKem™ plus 1% NuSieve™ gel (FMC BioProducts, Rockland, ME, USA) containing ethidium bromide. DNA from established laboratory strains of P. falciparum served as controls of PCR and enzyme digests. Gels were recorded by photography.

Results

Over the 18-month time period we collected 277 blood samples from patients with a microscopically confirmed P. falciparum infection acquired in Africa. The DNA from all blood samples was amplified by nested PCR systems as described above. 271 samples showed the expected bands at 522 bp for the DHFR gene, while six samples turned out to be negative for P. falciparum DNA.

Among the 271 observed samples of P. falciparum, 170 (62.7%) were imported from West Africa, 46 (16.6%) from East Africa, 36 (13%) from Central Africa, 17 (6.1%) from South Africa and two from Madagascar (Tab 1).

Table 1 Frequencies of geographical regions in which the observed Plasmodium falciparum infection has been acquired

After digestion none of the 271 isolates revealed the leucine-164 mutation. All 271 isolates of P. falciparum presented as the wild type with the amino acid isoleucine in position 164.

Discussion

The burden of falciparum malaria is carried mainly by tropical Africa. Since resistance to Chloroquine is now widespread in Africa, the antifolate combination sulfadoxine-pyrimethamine (SP) is often the only affordable alternative.

Unfortunately, SP is particularly prone to the rapid emergence of resistance [18]. There is evidence, that in vitro pyrimethamine chemosensitivity will be predictive of in vivo SP efficacy, and that parasitological resistance in Africa is primarily due to the triple mutant in DHFR: S108N, N51I and C59R [19–21].

At the moment, the only drug available in Africa to deal with SP failure is often further treatment with SP. Researchers are, therefore, seeking new, safe and inexpensive, second line drugs or combination regimens. Among these regimes chlorproguanil-dapsone is considered a potential replacement for SP in Africa because of its effectiveness against infections associated with triple mutants in DHFR, its short half-life and its low price [21–23]. However, the efficacy of combinations involving old drugs may be short-lived, since resistance-conferring mutations already exist. In the case of chlorproguanil-dapsone, the change in DHFR enzyme structure resulting from the I164L mutation is known to be associated with resistance to this drug [19, 20].

Experience with SP in South East Asia and South America has shown that continued use of SP will select for the quadruple mutant DHFR, including I164L. So far, mutations of this potency have not been observed in Africa in any study using standard PCR analyses [10, 17, 21, 23–27]. The I164L mutation was also not found in any of the isolates we analyzed within this survey.

Recently, Hastings et al. [28] developed a yeast-based system that is sensitive enough to identify point mutations in the DHFR gene even if they are rare. With the help of this technique alleles that encoded the I164L mutations were isolated from 3 of 6 patient samples with SP treatment failure from Tanzania. However, mutations of all kinds occur during DNA replication, but not all establish themselves and are functional.

Overall, efficient surveillance systems to detect and monitor resistance to SP are required, since malaria infections will become fully resistant to SP and chlorproguanil-dapsone with the selection of the DHFR mutation I164L in Africa. For this reason, several national and regional networks of sentinel sites have been set up in recent years and collaborate in order to monitor antimalarial drug resistance. The East African Network for Monitoring Antimalarial Treatment (EANMAT), for example, started in 1998 to monitor drug resistance with standard in vivo tests [29]. TropNetEurop, as the largest network for infectious diseases acquired by travellers worldwide, can function as an additional sensitive tool in the surveillance of antimalarial drug resistance.